1. Field of the Invention
The present invention relates to a waveguide antenna apparatus and an array antenna apparatus that employs the waveguide antenna apparatus, and in particular, to a waveguide antenna apparatus constituted by including a rectangular waveguide and an array antenna apparatus that employs the waveguide antenna apparatus.
2. Description of the Related Art
FIG. 59 is a perspective view showing a configuration of a prior art antenna apparatus (hereinafter referred to as a first prior art) that has a bi-directional pattern on the horizontal plane, and FIGS. 60A and 60B are graphs showing radiation directivity patterns of the antenna apparatus of FIG. 59. In this case, FIG. 60A is a graph showing a radiation directivity pattern on the X-Y plane, and FIG. 60B is a graph showing a radiation directivity pattern on the X-Z plane. In FIG. 59, reference is made to an X-Y-Z coordinates system illustrated for explanation. In this specification, the X-axis direction is referred to as an X-direction, a positive X-direction is referred to as a +X-direction, and a negative X-direction is referred to as a −X-direction. The similar thing can be applied to the Y-axis direction and the Z-axis direction.
Referring to FIG. 59, the first prior art antenna apparatus has a hollow housing formed by including a grounding conductor 112 of a bottom surface positioned on the X-Y plane, a linear ceiling conductor 111a and rectangular ceiling conductors 111b and 111c, which are arranged so as to oppose to the grounding conductor 111 and located on a top surface (hereinafter referred to as an antenna ceiling portion) of the antenna apparatus, and four side surface conductors 113a, 113b, 113c and 113d, which serve as the side surfaces of the antenna apparatus. The grounding conductor 112, the side surface conductors 113a to 113d and the ceiling conductors 111a and 111b are joined with each other so as to be electrically connected with each other, constituting a rectangular parallelepiped hollow housing symmetrical with respect to the Y-Z plane and the X-Z plane. In an approximate center portion of the antenna ceiling portion, a rectangular opening 116 is formed between the ceiling conductor 1111a and the ceiling conductor 111b, which extend parallel to the Y-axis, and a rectangular opening 117 is formed between the ceiling conductor 111a and the ceiling conductor 111c. With this arrangement, two rectangular opening spaces each having the same shape are arranged symmetrically with respect to the Y-Z plane in the antenna ceiling portion. One end of an antenna element 114 made of a conductor wire is connected mechanically and electrically with a connection point located in a center portion in the longitudinal direction of the ceiling conductor 111a by soldering or the like. Another end of the antenna element 114 is electrically connected with a central conductor (not shown) of a coaxial cable fed with a radio signal from a radio transceiver at a feeding point 115 in a circular hole 115h formed in a center portion of the top surface of the grounding conductor 112 (the center portion is located at an origin of the X-Y plane in the figure). At the feeding point 115, a grounding conductor of the coaxial cable is electrically connected with the grounding conductor 112.
In this case, a space, which is surrounded by the ceiling conductors 111a to 111c, the side surface conductors 113a to 113d and the grounding conductor 112, is referred to as an antenna interior, and a space on the outside of the antenna interior is referred to as an antenna exterior.
As an example, the grounding conductor 112 has a square shape of a side of 0.76 wavelengths with reference to a free space wavelength corresponding to the operation frequency, and the height of the side surface conductors 113a to 113d is 0.08 wavelengths. The antenna ceiling portion is constituted by including the ceiling conductor 111a made of one linear conductor and the ceiling conductors 111b and 111c made of the two rectangular conductors. The linear ceiling conductor 111a is arranged so as to be parallel to the Y-axis on the Y-Z plane and has a length of 0.76 wavelengths, and its both ends are electrically connected with the side surface conductors 113a and 113c. Both of the rectangular ceiling conductors 111b and 111c have a side parallel to the X-axis of a length of 0.19 wavelengths and a side parallel to the Y-axis of a length of 0.76 wavelengths. The conductors are arranged at both ends in the X-direction of the antenna ceiling portion, and are electrically connected with the side surface conductors 113a to 113d. The two opening portions 116 and 117 are formed so as to be a rectangle that has a side parallel to the X-axis of a length of 0.19 wavelengths and a side parallel to the Y-axis of a length of 0.76 wavelengths. The two opening portions 116 and 117 are arranged so as to be located to be adjacent to each other with interposition of the linear ceiling conductor 111a arranged in the center portion of the antenna ceiling portion, and the present antenna apparatus has a structure symmetrical with respect to the X-Z plane and the Y-Z plane. In this case, the antenna element 114 is made of a conductor wire, and the length of the antenna element 114 is 0.08 wavelengths. The antenna element is extended vertically so as to be perpendicular to the grounding conductor 112, and the top end portion of the antenna element 114 is electrically connected with the linear ceiling conductor 111a of the antenna ceiling portion in the center portion in the longitudinal direction of the ceiling conductor 111a. 
FIGS. 60A and 60B are graphs showing radiation directivity patterns of the antenna apparatus provided with the two opening spaces of the above-mentioned structure as one example. FIG. 60A is a graph showing a radiation directivity pattern on the X-Y plane, and FIG. 60B is a graph showing a radiation directivity pattern on the X-Z plane. With regard to the scale in the radial direction representing the gain of the antenna apparatus, one interval is 10 dB, and there is used a unit of dBd of the relative gain with reference to the gain of a dipole antenna. In the antenna apparatus of the monopole antenna shown in FIG. 59, radiation of an electromagnetic wave in the Y-direction is suppressed, and a bi-directional pattern in the +X-direction and the −X-direction is obtained. Therefore, the above-mentioned prior art example shows excellent characteristics when used in an elongated indoor space of a corridor or the like.
Moreover, in the present antenna apparatus, the opening portions 116 and 117 for radiating an electromagnetic wave are arranged in the antenna ceiling portion, and the antenna element 114 of a radiation source is surrounded by the grounding conductor 112 and the side surface conductors 113a to 113d. Accordingly, there is caused a little influence of the antenna arrangement environment in the sidewise and downward directions of the antenna on the radiated electromagnetic wave. That is, in the case where the present antenna apparatus is installed on an indoor ceiling or the like, it is possible to embed the antenna apparatus in the indoor ceiling and install the antenna apparatus in alignment with the indoor ceiling so that the ceiling portion of the antenna apparatus face the radiation space. With this arrangement, no projection is present on the ceiling or the like, and an aesthetically preferable antenna apparatus that attracts little human attention is provided.
Moreover, the height of the antenna element 114 is 0.08 wavelengths which is smaller than that of the normal quarter-wavelength monopole antenna element. As described above, according to the structure of the present antenna apparatus, there is provided an aesthetically preferable antenna apparatus that attracts little human attention with a small projection on the ceiling also by virtue of the advantageous effect of the low-profile configuration of the antenna element in the case where the antenna apparatus cannot be embedded in the indoor ceiling.
Furthermore, with regard to the present antenna apparatus of the above-mentioned first prior art, there has been described the structure symmetrical with respect to the Y-Z plane and the X-Z plane. In this case, there is such an advantageous effect that the directivity pattern of the electromagnetic wave radiated from the antenna apparatus becomes symmetrical with respect to the Y-Z plane and the X-Z plane. As described above, according to the present antenna apparatus, there can be provided a compact excellent monopole antenna apparatus that has a desired bi-directional pattern.
Moreover, as an array antenna apparatus provided with a plurality of sector antennas that have radiated strong main beam in one direction on the horizontal plane, there is, for example, the antenna apparatus described in the Japanese Patent Laid-Open Publication No. JP 9-135115 A (hereinafter referred to as a second prior art).
Referring to FIG. 1 of the publication of the second prior art, there is provided a three-dimensional corner reflector antenna apparatus constituted by including a grounding conductor at least the surface made of a conductor, at least one antenna element provided vertically so as to be perpendicular to the grounding conductor, side surface conductors provided on both sides of the antenna element and a reflector conductor provided behind the antenna element (i.e., in the direction opposite to the direction of radiation of a radio signal). The published antenna apparatus is characterized in that one or a plurality of fins at least the surface of which is a conductor is attached to the side surface conductors located on both sides of the above-mentioned antenna element. It can be understood that the three-dimensional corner reflector antenna provided with the conductor fin (metal fin) disclosed in the publication of the second prior art is able to sharpen the beam width of only the radiation directivity pattern on the horizontal plane by the advantageous effect of the electromagnetic field distribution control with the conductor fin while scarcely changing the configuration of the radiation directivity pattern on the vertical plane and the tilt angle.
However, the first prior art antenna apparatus shown in FIG. 59 has the following problems. As described hereinabove, the bi-directional pattern has been able to be obtained, whereas a directivity having an extremely strong main beam in one direction was not obtained. The first prior art antenna apparatus, which is suitable for an elongated coverage area of a corridor or the like, can not effectively radiate an electromagnetic wave in the case where the antenna apparatus is allowed to be installed only at the end of the coverage area such as a place near an indoor wall or window. That is, there has been a restriction in the installation place of the antenna apparatus. Therefore, the structure of the antenna apparatus of the first prior art, which has not been able to effectively utilize the electromagnetic wave radiated from the antenna apparatus in the case where the antenna apparatus has been allowed to be installed only at the end of the coverage area, has been inevitably regarded as improper.
Moreover, the antenna apparatus disclosed in the publication of the second prior art has the following problems. The second prior art antenna apparatus has an antenna height (i.e., the height of the side surface conductors and the reflection conductor) of 0.6 wavelengths and is not able to be regarded as a low-profile antenna apparatus. When the antenna is arranged on the indoor ceiling or the like, a compact low-profile configuration is desired so that the antenna does not attract human attention. For example, if the frequency of the radio signal to be transmitted and received is 900 MHz, then 0.6 wavelengths correspond to 198 mm. Assuming that the antenna apparatus is provided with a cover, the height becomes equal to or smaller than a height of at least 200 mm. Therefore, the structure of the second prior art, which has had a tendency to attract human attention because of its incapability of having a low-profile configuration, has been inevitably regarded as improper.